1. Field of the Invention
The present invention relates to techniques for encrypting/decrypting digital data transferred among computers, household information processing appliances, and electronic toll collection apparatuses.
2. Description of the Related Art
Enciphering techniques for preventing an illegal copy of digital data are essential for digital household information processing appliances. For example, if digital visual data received by a digital broadcasting receiver is digitally recorded in a digital video recorder and the digital visual data has a copyright, both the receiver and digital video recorder are required to have a function of protecting the copyright. In order to realize such a copyright protection system, it is necessary to prevent alteration and illegal copying of digital data by means of setting a limitation to digital data copying, device authentication, and cryptographic techniques such as real time cryptograph of digital data.
An example of conventional cryptographic techniques may be a symmetric key or common key algorithm, typically DES cryptograph disclosed in U.S. Pat. No. 3,962,539. Most of common key algorithms are characterized in a complicated cryptogram formed by repeating a simple conversion. Various approaches have been tried in order to improve security of cryptograms. For example, a cryptographic attack can be made difficult by increasing the number of repetitions of simple conversions to further disturb statistical characteristics of cipher texts.
However, if the number of conversion repetitions is increased, the processing time required for cryptographic conversion becomes long. Therefore, a security reinforcing countermeasure through an increase in the number of repetitions of simple conversions is not suitable for real time cryptograph in the copyright protection system.
In an electronic toll collection system (ETC) of a toll speed-way which has lately attracted attention, a real-time cryptographic processing is required, so that the problem as mentioned above arises.
The electronic toll collection system represents a system which is capable of collecting a toll based on an electronic transaction through a wireless communication between an antenna provided at a toll collecting station and an on-board equipment mounted on a car when the car passes through the toll collecting station, the details of which are described in for example, a Japanese magazine xe2x80x9cCard Wavexe2x80x9d published by C-Media, March, 1999, pp42-45. In the referred-to system, a real-time cryptographic processing is indispensable in order to send and receive exchange data at real time and protect the exchange data from bugging and unauthorized alteration.
It is an object of the present invention to provide an encrypting conversion apparatus, a decrypting conversion apparatus, a cryptographic communication system and an electronic toll collection apparatus capable of changing algorithms of cryptographic conversion to hide the algorithm in use from a third party so that the apparatuses and system are resistant against a cryptographic attack and can operate at high speed.
According to one aspect of the present invention, there is provided an encrypting conversion apparatus for inputting at least one cipher key, at least one algorithm parameter, and plain text data and outputting cipher text data, the apparatus comprising: a plurality stage of encrypting conversion means for executing each of an exclusive logical sum operation, a cyclic shift operation and an addition operation at least once, wherein: the encrypting conversion means includes at least one of each of first to third operation means, the first operation means executes either an exclusive logical sum operation or an addition operation of input data and a portion of data generated from data of the cipher key, the second operation means executes either an exclusive logical sum operation or an addition operation of input data and a portion of data determined by the algorithm parameter, and the third operation means cyclically shifts input data by the number of bits determined by the algorithm parameter; and conversions which use combinations of a plurality stage of consecutive encrypting conversion means optionally selected from all of the encrypting conversion means and use the same input data and the same algorithm parameter, are all different.
According to another aspect of the present invention, there is provided a decrypting conversion apparatus for inputting at least one cipher key, at least one algorithm parameter, and cipher text data and outputting plain text data, the apparatus comprising: a plurality stage of decrypting conversion means for executing each of an exclusive logical sum operation, a cyclic shift operation and an addition operation at least once, wherein: the decrypting conversion means includes at least one of each of first to third operation means, the first operation means executes either an exclusive logical sum operation or an addition operation of input data and a portion of data generated from data of the cipher key, the second operation means executes either an exclusive logical sum operation or an addition operation of input data and a portion of data determined by the algorithm parameter, and the third operation means cyclically shifts input data by the number of bits determined by the algorithm parameter; and conversions which use combinations of a plurality stage of consecutive decrypting conversion means optionally selected from all of the encrypting conversion means and use the same input data and the same algorithm parameter, are all different.
According to another aspect of the present invention, there is provided a cryptographic communication system with a common key algorithm for communication between a transmitter apparatus and a receiver apparatus having a same cipher key, the transmitter apparatus encrypting a plain text by using the cipher key to acquire and transmit a cipher text, and the receiving apparatus decrypting the received cipher text by using the cipher key to recover the plain text, wherein: the transmitter apparatus includes encrypting conversion means and first algorithm key storing means; the receiver apparatus includes decrypting conversion means and second algorithm key storing mean; a conversion algorithm to be executed by the encrypting conversion means of the transmitter apparatus is determined by a first parameter stored in the first algorithm key storing means of the transmitter apparatus; a conversion algorithm to be executed by the decrypting conversion means of the receiver apparatus is determined by a second parameter stored in the second algorithm key storing means of the receiver apparatus; and the cipher text encrypted by the transmitter apparatus by using the cipher key can be correctly decrypted by the receiver apparatus by using the cipher key, only if the cipher key as well as the first and second parameters used by the transmitter and receiver apparatuses is same.